Porous metal-alumina composite

ABSTRACT

A porous composite structure comprising a sintered mixture of metal particles and alumina particles, and a method for producing this composite. The composite is useful as a catalyst support, particularly in apparatus designed to reduce pollutant emissions from automobile engines.

This application is a division of our prior U.S. application Ser. No.698,030, filing date June 2, 1976, now U.S. Pat. No. 4,058,485, which isa continuation of application Ser. No. 536,646, filing date Dec. 26,1974, now abandoned.

BACKGROUND

Porous metal structures, ceramic monoliths and the so-called cermets,that is, composites of ceramic and metal materials, have been known formany years. These materials have been useful because of their inherentcatalytic properties as well as their ability to serve as catalystsupport structures; for example, porous nickel sheets have been used inbattery electrodes, particularly as components in fuel cell electrodes,while ceramic and cermet materials have been used as supports for metalsand metal oxides which are active as catalysts in chemical reactions. Inrecent years there has been increasing interest in the use of ceramicand cermet materials as supports for catalytic materials used intreating automobile exhaust emissions. See for example U.S. Pat. No.3,444,925 to Johnson which discloses ceramic and cermet honeycombstructures and their use in automobile exhaust gas catalytic converters;U.S. Pat. No. 3,492,098 to De Palma et al. in which a multi-layerstructure, which includes a ceramic layer and an alumina layer, is usedas a support for a third layer of catalyst; and U.S. Pat. Nos. 3,471,413and 3,492,148 both to Hervet which also disclose multi-layer structuresincluding a porcelain layer, an alumina layer and a catalyst layer.

The ideal catalyst support should combine physical strength, highporosity to permit rapid fluid transport to and out of the support, ahigh surface area on which catalytic materials can be deposited andability to withstand elevated operating temperatures. The porouscatalyst supports heretofore known have often been satisfactory as tosome of these properties, but none of the catalyst supports heretoforeknown have been totally satisfactory for uses under stringent operatingconditions, for example, in auto exhaust catalytic converters.

The present invention provides an improved porous composite structurewhich exhibits all of the aforementioned desirable properties ofstrength, internal porosity, high surface area and ability to withstandelevated temperature service.

IN THE DRAWINGS

FIG. 1 is a graph showing the effect of heating time and heatingtemperature on the surface area of alumina.

FIG. 2 shows a typical exhaust gas catalytic converter which employesthe composite structure of this invention.

FIG. 3 shows another exhaust gas catalytic converter structure whichemploys the composite structure of this invention.

SUMMARY OF THE INVENTION

The present invention includes a porous composite structure comprising asintered mixture of metal particles and alumina particles characterizedby the following parameters: (a) at least 50 weight percent of theparticles in the composite are metal particles; (b) at least 5 weightpercent of the particles in the composite are alumina particles having asurface area of at least 25 square meters per gram; and (c) theinter-particle porosity in the composite structure is between 5 volumepercent and 60 volume percent. In a preferred embodiment particularlyuseful as a catalyst support, the inter-particle porosity is at least 15volume percent. In another preferred embodiment, a metal screen, grid,mesh or perforated metal sheet is used to give the composite structureadditional mechanical strength.

The invention also includes a method for producing a porous sinteredmixture of metal particles and alumina particles comprising the stepsof: (1) forming a mixture of (i) particles of a metal having a sinteringtemperature below the phase transition temperature for the formation ofalpha-alumina and (ii) transition alumina particles having a surfacearea sufficiently high so that they will retain a surface area of atleast 25 square meters per gram after sintering of the metal particlespursuant to step (3) below, such mixture containing at least 50 weightpercent metal particles and at least 5 weight percent (on an anhydrousbasis) transition alumina particles; (2) compacting the mixture into adesired shape under a pressure high enough so that the compacted mixturewill retain its shape, but low enough so that the mixture aftersintering will have an inter-particle porosity of at least 5 volumepercent; and (3) heating the compacted mixture to a temperature belowthe phase transition temperature of transition alumina to alpha-aluminafor a sufficient length of time to cause sintering of the metalparticles.

DETAILED DESCRIPTION

As used herein, the term "intra-particle porosity" means the volumepercent of void space within an individual particle, and the term"inter-particle porosity" means the volume percent of void space withina given volume of the sintered composite structure of the invention.Inter-particle porosity does not include the intra-particle porosity ofparticles within the given volume of the composite structure. Bothintra- and inter-particle porosity can be measured by the methodsdescribed in "Adsorption, Surface Area and Porosity", by S. J. Gregg andK. S. W. Sing, Academic Press, New York, 1967.

As used herein, the term "transition alumina" means the substantiallyanhydrous form of the aluminum oxide obtained by heating an aluminumoxide hydrate to temperatures above about 500° C. Aluminum oxidehydrates and transition aluminas are described in detail inKirk-Othmer's Encyclopedia of Chemical Technology, 2nd Edition, Vol. 2,pp. 42-50, 1963.

As used herein, the term "sintering" means the bonding together attemperatures below their melting temperatures of a sufficient number ofparticles in a particulate mass to impart three-dimensional structuralcoherence to the mass.

As used herein, the term "scaling" means the susceptibility of a metalto change weight in a corrosive atmosphere and is a measure of oxidationresistance, while the term "scaling temperature" means the temperaturebelow which the rate of oxidation is less than 0.002 gm/in² /hr, asspecified in "Corrosion Engineering", by M. G. Fontana and N. D. Green,McGraw-Hill, New York, 1967, page 369.

The composite of this invention is a porous composite body of metal andhigh surface area alumina. The composite is formed by blending metalparticles and alumina particles into a uniform mixture, compacting themixture at ambient temperatures, and then sintering the green substratein a reducing atmosphere for specified times and temperatures. Theresulting porous composite structure is a mechanically tough, porous,high surface area material.

A principal use of the porous composite structure of this invention isas a catalyst support substrate, and the following discussion includes adetailed description of such use with particular reference to use inapparatus designed to reduce pollutant emissions from automobileengines.

The typical automobile equipped with an internal combustion (IC) engineemits an exhaust gas that contains three pollutants; unburnedhydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO_(x)).The Environmental Protection Agency (EPA) has set maximum limits forthese pollutants that will have to be met by new automobiles within thenext few years.

One promising way of meeting the new standards is through the use ofcatalytic converters, that is, devices which convert the pollutants inautomobile exhaust into relatively harmless substances. To react the HCand CO pollutants to the end products of carbon dioxide and water, it isnecessary to maintain an oxidizing atmosphere, whereas to react thenitrogen oxides to nitrogen, it is advantageous to maintain a reducingatmosphere. Further, in order to economically perform the reactions atthe temperatures that normally exist in the auto exhaust system, it isnecessary to utilize catalysts to increase the desirable reactionsrates. Since the desirable conditions for the pollutants are different,it has been proposed that the post-treatment of the exhaust gases beperformed in two stages. The first stage would catalytically react theNO_(x) to N₂, whereas the second stage would react an auxiliary airstream and the HC and CO pollutants. Such a two-stage system would takemaximum advantage of desirable equilibrium conditions for the reactions,that is, for the NO_(x) conversion, a predominantly reducing atmosphereand possibly lower temperatures, and, for the HC and CO conversion,relatively high temperatures and an oxidizing atmosphere. By maintainingmore favorable conditions for each stage, it is easier to find suitablecatalysts for each reaction that will increase the overall reactionrates to the necessary levels. It is evident that a single stage systemto treat all three major pollutants could be used if suitable catalystsbecomes readily available.

For all reactions aided by catalysts, it is necessary that the reactantfluids be brought into contact with one another in the presence of thecatalyst. Only then can the catalyst perform its functions of increasingthe reactant kinetics. To prevent mass transfer limitation, it isdesirable to distribute the active catalyst over a wide surface area toaid reactant fluid contact. Although it is possible for the catalystitself to be its own support, this is not usually the case, due todifferent desirable properties for the catalyst and catalyst support anddue to the higher cost of most catalysts compared to catalyst supportmaterials. The above-described catalytic converters for control of autoexhaust emissions can advantageously use a catalyst support composed ofmetal and transition alumina that is an embodiment of this invention.The catalyst support has the desirable features of high specific surfacearea, good temperature resistance, and good mechanical strength.

Each of the two components of the catalyst support substrate of thisinvention has a definite purpose and must meet the requirements for thatpurpose. The metal component of the substrate is primarily intended toimpart mechanical strength to the substrate, whereas the aluminacomponent is primarily intended to supply the high surface area for thecatalyst. The entire structure must be sufficiently porous to allowfluid transfer within the substrate and the entire structure must beserviceable at operating temperatures in an operating atmosphere.

The mechanical strength required for the catalyst support structure isprimarily good vibration and impact strength, with some tensilestrength. The first two criteria are especially valuable for automotiveapplications and somewhat less important for stationary applications.The tensile strength factor is an additional measure of the durabilityof the composite, in that it gives an indication of the ability of thecomposite to resist factors such as fluid erosion and fluid pressureloads. The degree to which the above-described properties are requireddepends to some extent on the physical form of the substrate, forexample, flat sheets, wound sheets, pellets and the like.

The metal component of the substrate must supply the mechanical strengthto the substrate and be able to maintain that strength at serviceconditions. For the auto emission control application, this means thatthe metal must be able to withstand frequent and repetitive applicationsof high service temperatures and oxidizing (and/or reducing) atmospheresthat contain water vapor. The ability to retain strength at hightemperatures is important because operation of the catalytic converterat higher temperatures aids the performance of the catalyst and resultsin the overall system being more competitive. That is, the performanceof the catalytic converter is the combined result of the conversion thatwould be obtained at the given temperature without the catalyst and theadditional conversion due to catalytic action. Thus, if the system hasthe capability of operating at higher steady-state temperatures, theoverall performance of the system would be enhanced. Additionally, themetal should be able to withstand high service temperatures forrelatively short durations that may result from temporary upsets of thesystem. For the auto catalytic converter, the expected steady-stateoperating temperatures are about 315° to 650° C. with possibleshort-term (less than 5 minutes) temperature excursions to 1100° C.,depending, of course, on the type of engine, fuel, and engineefficiency. Accordingly, the metal component of the catalyst supportsubstrate should be acceptable for those temperature ranges.

The various metal powders differ in their resistance to scaling atelevated temperatures as illustrated in the following Table 1. Thescaling temperatures for four different metals are listed in Table 1,the data being obtained from Fontana, M. G. and Greene, N. D., CorrosionEngineering, McGraw-Hill, New York, 1967, page 369.

                  Table 1                                                         ______________________________________                                        Metal            Scaling Temperature (° C.)                            ______________________________________                                        310 Stainless Steel                                                                            1149                                                         304 Stainless Steel                                                                            899                                                          Nickel           788                                                          1010 Carbon Steel                                                                              482                                                          ______________________________________                                    

It should be noted that the metal component imparts good thermal shockresistance to the substrate. This factor is important in the cyclicaltemperature environment of the automotive application and wouldfavorably influence the operating life of the composite structure. Thisis a definite advantage of the metal-alumina composite structurecompared to the all-ceramic system which would have lower resistance tothermal shock.

Another factor that determines the serviceability of the metal in thecatalyst support substrate is its resistance to the oxidizing (orreducing) atmosphere present in the second (first) stage of the autocatalytic converter. The second stage of a catalytic converter is oftenexposed to an auxiliary air supply (excess oxygen) and the highesttemperatures in the system (up to 650° C.), and is therefore arelatively severe service condition.

It is evident from the above discussion that two factors that determinethe suitability of a metal for use in the catalyst support substrate isits service temperature and resistance to oxidizing (or reducing)atmospheres. Additionally, a third factor is the ability of the metal tobe sintered at relatively low temperatures (less than about 1050° C.).This factor is related to the susceptibility of the high surface areaalumina component to high temperatures. As will be discussed in moredetail hereinbelow, transition alumina loses surface area upon exposureto high temperature.

Based on the above-described factors, suitable metals and metal alloysfor the catalyst support substrate include chromium, copper, cobalt,nickel alloys, iron alloys, Monel (67 Ni, 30 Cu, 1.4 Fe), Inconel (79.5Ni, 13 Cr, 6.5 Fe), Stainless Steel 304 (19 Cr, 9.5 Ni, Fe), Nichrome(80 Ni, 20 Cr), Stainless Steel 310L (25 Cr, 20.5 Ni, Fe), 6% Al-Fe (6Al, Fe), and the like. The above list is only representative of suitablemetal components for the catalyst support substrate and other metals canbe suitable, particularly where the catalyst support substrate is to beused for applications other than the catalytic converter for autoemission control so that service conditions are less severe.

The primary purpose of the alumina component in the catalyst supportsubstrate is to supply a large surface area for catalyst deposition. Thealumina component supplies relatively little mechanical strength to thesubstrate, but it has good resistance to the system atmosphere. Anespecially suitable type of alumina for the catalyst support substrateare the forms of aluminum oxide known as the transition-alumina. Theterminology associated with various forms of alumina has not beenentirely consistent. Accordingly, the following description of thepreparation and properties of transition-alumina is based on theKirk-Othmer reference cited hereinabove. Typically, a transition aluminacan be formed by starting with the gel-type alpha monohydrate(gel-Boehmite) formed and then heating it to drive off the water.Alumina gels lose their water of constitution progressively on heatingfrom about 150° C. upwards, but the last few percent is not eliminateduntil about 500° C. The resulting transition alumina has a relativelyhigh internal porosity (about 50 volume %) and pore diameters (about 50to 100 Angstroms) that results in large internal surface area rangingfrom 200 to 400 m² /gm which, even after heating in the process of thisinvention retains a substantial internal surface area available forcatalyst deposition and a pore structure that offers relatively littleresistance to mass transfer. Such materials are highly desirable for usein the process of this invention in that they allow the ready diffusionof reactants into the structure and permit good utilization of theinternal area. Transition alumina is susceptible to temperature andundergoes a phase transition at about 1150° C. into the alpha-alumina orcorundum form. The alpha-alumina form is a relatively dense (less than1% porosity) material that has very low internal surface area (less than1 m² /gm). The properties of the alpha-alumina are thus undesirable as acatalyst support.

Accordingly, it can be seen that the expected exposure temperature forthe catalyst support substrate, both during sintering and subsequentuse, is important to avoid conversion of the transition alumina into thecorundum form, the primary drawback of this conversion being the loss ofalumina surface area. The effects of heating and of this conversion areshown in FIG. 1.

FIG. 1 shows the reduction in available surface area as a function ofheating time at various temperatures for a typical commercial gradetransition alumina having a surface area of 322 m² /gm prior to heating.Samples of the alumina were heated in air at the temperatures and forthe time periods shown by the data points set forth in FIG. 1, and thesurface area was measured after cooling the samples to room temperature.Smooth curves were then drawn through the data points; curve A shows thechange in surface area upon heating for 15 minutes, curve B for heatingover a period of 1 to 4 hours and curve C for 18 hours of heating. Ascan be seen from FIG. 1, the effective surface area of the alumina issharply reduced by extended heating at temperatures approaching thephase transition temperature (1150° C.) from transition alumina toalpha-alumina. These data also show that where sintering temperaturesfor formation of the porous composite material and its surfacetemperature in actual use are below about 1050° C., a relatively highsurface area is retained even upon extended periods of heating. Someloss of alumina surface area during sintering and subsequent use atelevated temperatures is unavoidable. However, the retained surface areaof the substrate of this invention is high relative to prior artstructures and is readily accessible because of the ease of masstransfer throughout the substrate.

A number of transition alumina materials are commercially available. Thecomposition and properties of two typical transition aluminas are setforth in Table 2.

                  Table 2                                                         ______________________________________                                        Chemical Analysis, %                                                                           Alumina #1  Alumina #2                                       ______________________________________                                         Al.sub.2 O.sub.3                                                                              85          69.7                                              SiO.sub.2       5.8         5.7                                               Na.sub.2 O      2.0         0.04                                              Fe.sub.2 O.sub.3                                                                              0.10        0.02                                              Ignition Loss (H.sub.2 O)                                                                     6.0         23.8                                             Surface Area     350         239                                               m.sup.2 /gm (ambient)                                                        Pore Volume      0.5         0.3                                               (ml/gm)                                                                      Pore Diameter    60          --                                                (A°)                                                                  ______________________________________                                    

Many transition aluminas, such as those of Table 2, contain a fewpercent silica which is in no way detrimental in the process orcomposite of this invention. Transition alumina with high surface areasuch as those shown in Table 2 are preferred; however, any transitionalumina material can be used in the practice of this invention so longas it has an initial surface area sufficiently high to retain a finalsurface area after sintering of at least 25 m² /gm, and preferably atleast 50 m² /gm. As a general rule, the surface area of the transitionalumina prior to sintering should be at least 200 m² /gm. Surface areameasures can be made by conventional adsorption techniques; for example,that described in Gregg, S. J. and Sing, K. S. W., "Adsorption, SurfaceArea, and Porosity", Academic Press, 1967, New York, page 49.

In addition to silica, the alumina component of this invention cancontain, or can be mixed with, other metal oxides which are inert orcontribute to the total catalytic activity of the composite structure.For example, if the alumina is treated with catalyst prior to sintering(a procedure described in detail herein below), the alumina may containup to a few percent by weight of oxides of the catalytic metals.Alternatively, the alumina can be mixed up to 5% to 45% by weight ofinert metal oxides such as silica or zirconia. These inert additions arenot detrimental to the composite structures of this invention so long asthe sintered composite contains at least 5 weight percent aluminaparticles having a surface area of at least 25 square meters per gram.

A potential problem encountered with the composite structures of thisinvention is that of maintaining high catalytic activity of loadedsubstrates upon extended exposure to automotive exhaust conditions. Theloaded substrates exhibit high initial activity, but tend to degradeduring service. It has been discovered, however, that pretreating thealumina component under severe conditions of heat and moisturesubstantially eliminates the problem and results in a reactor havinghigh stable activity.

A typical silica-stabilized transition alumina has, in the as-receivedcondition, a surface area of 272 m² /gm. After the pretreatmentprocedure described below, the surface area is typically reduced toabout 150-170 m² /gm. However this remaining surface area is stable;exposure of the pretreated alumina to sintering and/or serviceconditions does not produce significant continued degradation.

Pretreatment for automobile exhaust service consists of subjecting thealumina to a temperature of 866°-927° C. in an atmosphere containing10-15% water for 16-24 hours. These conditions are more severe thanthose normally encountered in automotive exhaust service.

One suitable pretreatment procedure consists of two steps: first,heating the alumina in hydrogen at 927° C. for 3 hours and second,reheating in hydrogen plus 10% water at 866° C. overnight. The followingsurface area measurements show the result:

    ______________________________________                                               Surface Area                                                                             Surface Area Surface Area                                          As received,                                                                             After First Step                                                                           After Second Step                              Alumina                                                                              m.sup.2 /gm                                                                              m.sup.2 /gm  m.sup.2 /gm                                    ______________________________________                                        #2 of  272        181          161                                            Table 2                                                                       #1 of  --         184          133                                            Table 2                                                                       ______________________________________                                    

In a modified procedure, satisfactory results are obtained bysubstituting nitrogen for hydrogen. In one series of tests on thesilica-stabilized alumina (#2 of Table 2), the first step using hydrogenas described above was retained, but nitrogen was substituted in thesecond step. In order to confirm that the surface area was stable, thesecond step was repeated twice (3 steaming steps in all), each lasting16 hours. Surface area measurements were as follows:

    ______________________________________                                                            Subsequent                                                Operation           Surface Area, m.sup.2 /gm                                 ______________________________________                                        As received         about 272                                                 3 hrs. at 927° C. in H.sub.2                                                               181                                                       16 hrs. at 871° C. in N.sub.2                                          + 10-15% H.sub.2 O  162                                                       16 hrs. at 871° in N.sub.2                                             + 10-15% H.sub.2 O  152                                                       16 hrs. at 871° C. in N.sub.2                                          + 10-15% H.sub.2 O  147                                                       ______________________________________                                    

The two additional steaming operations caused only about 9% additionalsurface area reduction which is not considered significant. One periodof steaming is sufficient.

In a further modification, the two-step procedure can be combined in onestep consisting of 24 hours heating at 927° C. in N₂ +10-15% H₂ O. Finalstabilized surface area was 156 m² /gm (initial surface area 272 m²/gm).

Pre-treatment temperatures of 866° to 927° C. were selected for aluminato be used in automotive exhaust service because such condition is atleast as severe as that expected in actual service. Where the compositestructure is to be used in less severe service, for example, in stackgas purification, lower pre-treatment temperatures can be used, whichresults in somewhat higher stabilized surface areas.

An atmosphere containing air or oxygen can also be used in thepre-treatment procedure. However, inert gases such as hydrogen andnitrogen are preferred to prevent oxidation damage to metal furnacecomponents.

In summary, the pre-treatment method of this invention comprisescontacting transition alumina with water vapor at elevated temperaturesfor a period of at least three hours, the elevated temperature being atleast as high as the temperature at which the composite structure ofthis invention is expected to be subjected during its intended use.Temperatures in the range of 600° C. to 1000° C. are generallysatisfactory and do not result in undesirable loss of surface area.Where the substrate is intended for use in automobile exhaust service,pre-treatment temperatures in the range of 800° C. to 1000° C. arepreferred.

It is generally easier to carry out the pre-treatment method of thisinvention on the transition alumina before it is incorporated into thesintered composite structure. However, the pre-treatment can be carriedout on a finished composite structure after sintering with substantiallythe same results.

The pre-treatment of the alumina should be completed before loading acatalyst thereon. If the alumina is first catalized and thenpre-treated, some of the pre-deposited catalyst is encapsulated orentrapped or otherwise loses it activity. When the alumina is firstpre-treated, the catalyst is then applied to the stabilized surface ofthe alumina and remains freely accessible and active.

Although the above discussion has outlined the required characteristicsof the metal and alumina components to be used in preparing the catalystsupport substrate, there are certain additional factors which must beconsidered in order to obtain the most effective catalyst supportsubstrate material. Mechanical tests have shown that at least 50 weightpercent of the above-described metal component should be present forgood mechanical strength. As can be seen in Table 3, the use of allmetal leads to a very high strength, whereas high alumina is very lowstrength. It is necessary to have sufficient metal component to provideat least partially continuous frameworks which result in good mechanicalstrength for the substrate. Experimental testing of two typical metalcomponent and transition alumina substrates has indicated that thesubstrate strength and ductility are inadequate below about 50 wt. % ofmetal component. Such weak substrates would have relatively lowresistance of fluid erosion and vibration loads that characterize severesubstrate applications such as automobile catalytic converters.

                  Table 3                                                         ______________________________________                                          The porous sintered composites tested were                                  prepared by mixing nickel and transition alumina powders                      (both minus 325 mesh), compacting the mixture into flat                       sheets 1.27 cm. × 5.08 cm. × 0.5 mm. followed by sinter-          ing in a hydrogen atmosphere for 6 hours at 870° C. The                tensile strength measurements were made by suspending a                       weight pan from each of the flat sheets and progressively                     adding weights until breakage occurred.                                                          Tensile Strength                                           Wt.-% Metal        (kg/cm.sup.2)                                              ______________________________________                                        50                  67                                                        75                 120                                                        90                 239                                                        100                3818                                                       ______________________________________                                    

Calculations and tests have shown that a catalyst support substrateshould preferably contain at least 10 weight percent transition aluminacomponent. This quantity is based on the requirement of sufficientsurface area in the substrate to support the active catalyst, and is ofcourse related to the available surface area of the transition alumina,since less of a high surface area material will supply the same totalsurface area within the substrate that would be available from a largerquantity of lower surface area transition alumina. Additionally, theweight limit of transition alumina (and thereby a significant portion ofthe total available surface area) in the substrate is related to theexpected activity of the catalyst that will be used with the catalystsupport. With a high-activity catalyst (such as the noble metals,platinum or palladium), less catalyst will be required than withrelatively low-activity catalysts (such as oxides of the base metalssuch as, Cr₂ O₃, CuO, MnO, either alone or doped with trace quantitiesof the noble metals), for similar improvements in reaction rates. Forequivalent distribution of the catalyst on the catalyst support surface,it is evident that systems that require more catalyst will require moresurface area. These different requirements for diverse catalysts can beaccommodated by varying the quantity of transition alumina in thecatalyst support substrate. At least 5 weight percent transition aluminais required in the substrate in order to provide adequate surface arearegardless of the activity of the catalyst component. For onlymoderately active catalysts at least 10 weight percent transitionalumina and preferably 30 weight percent transition alumina should beincluded in the substrate.

As mentioned previously, one factor that is a variable in themanufacture of the catalyst support substrate are the sinteringconditions. The sintering operation can be carried out in vacuum or inany atmosphere which does not react unfavorably with the substratecomponents or catalytic components, and an inert or reducing atmosphereis preferred. Hydrogen provides a convenient reducing atmosphere, whilethe noble gases such as argon and neon provide convenient inertatmospheres. Nitrogen gas also provides a non-reactive atmosphere withmany substrate systems. The sintering temperatures and heating times arealso variable. Tests were performed on a nickel-transition aluminasystem and are shown in Table 4. The sample preparation and tensilestrength tests were carried out in the same manner as described inconnection with Table 3 hereinabove, except for the differentcombinations of heating temperature and duration shown in Table 4. Thedata in Table 4 illustrate the typical effect of sintering temperatureand time on the tensile strength of porous composite structures of thisinvention.

It should be noted that, at similar conditions, the thicker specimens(0.5 mm) exhibited better tensile strength than the thinner (0.23 mm)specimens.

                                      Table 4                                     __________________________________________________________________________    System       Nickel-Transition Alumina                                        Specimen Size                                                                              1.27 cm. × 5.08 cm. × 0.23 mm.                                                   1.27 cm. × 5.08 cm. × 0.5 mm.         Metal Content (wt-%)                                                                       100 90 75 50   100 90  75  50                                    Sintering Condition                                                           Temperature                                                                           Time                                                                  (° C.)                                                                         (Hr.)                                                                              Tensile Strength (kg/cm.sup.2)                                                               Tensile Strength (kg/cm.sup.2)                    __________________________________________________________________________    871     1    3340                                                                              105                                                                              108                                                                              Brittle                                                                            --  --  --  --                                    871     6    3818                                                                              239                                                                              120                                                                               67  --  --  --  --                                    927     1    3178                                                                              433                                                                              96  92  --  313 218 136                                   982     1    3340                                                                              262                                                                              127                                                                              123  --  823 276 227                                   982     6    --  -- -- --   --  928 513 205                                   1093    1    3670                                                                              527                                                                              137                                                                              101  --  752 347 267                                   __________________________________________________________________________

In general, higher temperatures and longer exposure times are favorableto high substrate strength as shown by tensile tests. Further, to someextent, the effects of sintering temperature and time involve atrade-off in that combinations of higher temperatures and shorter timeperiods are equivalent to lower temperatures and longer exposure times.From a mechanical strength standpoint, high temperatures and longexposure times are most desirable. However, from the standpoint ofpreservation of the surface area of the transition alumina, lowtemperatures and short exposure times are best. The opposing criteriarequire a compromise that can be easily experimentally determined by ashort series of tests such as those of Table 4 for each metalpowder-transition alumina powder system. For example, it has been foundthat sintering conditions of 870° C. at 6 hours is favorable for thenickel-transition alumina system and 925° C. at 3 hours is favorable forthe Stainless Steel 310-transition alumina system.

The sintering operation should be carried out at low enough temperaturesand/or short enough periods of time so that the metal particles do notfuse together, as opposed to sintering. Fusion causes the open matrix tocontract and reduces the size of the openings and channels in the porousstructure with a resulting increase in density, a decrease in porosity,and a decrease in the ease of a fluid mass transfer. When a compositecontaining about 50 weight percent metal particles is properly sinteredas described above, the metal particles will comprise only about 20percent of the total volume of the completed substrate.

Another factor that needs to be considered in the makeup of the catalystsupport substrate is the inter-particle porosity of the substrate. Theintra-particle porosity of the transition alumina is about 40-70%,whereas the intra-particle porosity of the metal component isnegligible. This is satisfactory and desirable since the metal particleis intended to add strength, whereas the transition alumina particle isintended to add surface area. Once the reactant fluids are present atthe outer boundary of the transition alumina particle, the internalporosity allows satisfactory mass transfer within the pores to reach theactive catalyst on the internal surface area. However, it is evidentthat for a uniform catalyst support substrate, not all transitionalumina particles will be at the outer boundary and in direct contactwith the reactant fluids. In order for transition alumina particles ofthe support substrate to be active throughout the entire depth ofpractical thicknesses, it must be possible for the reaction systemfluids (both reactants and products) to be able to be transferredthrough the substrate. It is a very desirable characteristic of thecomposites of the invention that the catalyst support substrate hassubstantial inter-particle voids or porosity to allow for fluidtransfer, in contrast to many known substrates which relied upon asingle particle thickness (often referred to as a "wash coat") of highsurface area alumina on a relatively non-porous support. On the basis ofcalculations and tests, it has been found that the inter-particle voidsor porosity should be at least 5% and more preferably at least 15 vol.%. The higher the porosity, the easier it is to transfer fluids withinthe substrate and greater thickness of the substrate can be effectivecatalyst supports. However, the more porous substrates tend to havelower mechanical strength, and their gross void space also constitutesmore of the total composite volume which could otherwise be occupied bycatalysts or reaction-promoting material. Accordingly, a compromise mustbe made between the two requirements. It has been found that about 25vol. % inter-particle porosity represents an effective compromise formany systems.

For a reasonable approximation of inter-particle porosity, the densityof the metal particles can be assumed to be that of solid base metal andthat of the transition alumina particles, a value characteristic of thespecific alumina employed. Knowing these densities, the volume occupiedby each component can be determined from the known weight of eachcomponent in the finished substrate. If the volume of the finishedsubstrate is V_(s), then the inter-particle porosity ε is ##EQU1## whereV_(M) and V_(A) are metal and alumina volumes, respectively.

For a more broadly-applicable determination of inter-particle porosityone may employ the well-known Mercury-displacement porosimeter test,wherein the quantity of Hg capable of being held within the sinteredmatrix is found from the net displacement of Hg by an immersed sample ofthe matrix. The test should be conducted under substantial pressure,e.g. 2500 psi in order to compress entrapped gases to a negligiblevolume.

The optimum substrate porosity is related to the particular reactionsystem and catalyst effectiveness. The reaction mechanism with acatalyst is a series process. First, the reactants must be transported(by gas phase diffusion in the auto emission case) to the surfacecontaining the active catalyst. Secondly, the reaction takes place, andthirdly, the products are transported (again by diffusion) back to thebulk fluid stream. The overall process rate is controlled by the rate ofthe slowest step. The ideal catalyst support substrate is one which hasmass transfer characteristics such that the resistance to fluid transferto inner pores containing catalyst is low and thereby the catalyticreaction step is rate-controlling. This can be more closely approachedwith sufficient porosity in the substrate to allow easy transfer offluids, generally, at least 5 vol. % inter-particle porosity. The lowervalues will tend to be satisfactory for slow reactions, such as removalof sulfur from flue gases by an acceptor process, whereas, the higherporosity is desirable for fast reactions such as the oxidation ofhydrocarbons or carbon monoxide in auto emission control. Additionally,it should be noted that the required substrate porosity is related tothe substrate thickness. If it is desired to use thick, relativelymassive substrates (as in pellet or other form, such as flat plates),the required porosity will be higher than if the substrate is used inthinner sections. This inter-relationship can be seen from the followingsimplified equation for steady-state gas phase diffusion. ##EQU2## whereN--gas transferred, gm. moles/hr.

A--surface area, cm²

ε--inter-particle porosity, expressed as fraction of surface areaavailable for gas transfer

Dg--gas phase diffusion coefficient, cm² /hr.

ΔC--concentration driving force for gas transfer, gm. moles/cm³

ΔX'--substrate thickness through which gas must be transferred to reachthe alumina, cm.

The substrate thickness (i.e., pellet diameter for packed catalyst beds)will depend upon the various significant system design requirements,such as pressure drop, mass transfer, reaction rate, and requiredmechanical strength. In some severe applications mass transferrequirements may dictate a thin cross-section.

The composite structure porosity is basically related to componentparticle size and compaction pressure. If rigid spheres are uniformlypacked, the arrangement has a calculable porosity. However, the metaland transition alumina particles are not regular rigid spheres. Instead,they are powders that have at least one dimension less than a certainsize as determined by grading through standard sieves. Further, thetransition alumina particles are weaker than the metal particles andupon compaction, the transition alumina particles will crush andsubdivide. Also, since the particles are non-uniform, gravity settlingwill result in a certain porosity that can be reduced by compaction ofthe uniform blend of the two-component system. Initial compaction willbreak and crush the particle edges, whereas continued compaction willcrush and subdivide transition alumina particles. The resultantinter-particle porosity of the system is determined both by initialparticle size and compaction conditions. As a general rule for a givencompaction pressure, the inter-particle porosity decreases withdecreasing particle size only if the compaction pressure is less thanthe crush strength of the transition alumina particles.

An additional factor that must be considered for some applications ofthe composite of the invention as a catalyst support substrate is thespecific heat capacity of the composite. For those applications that areintermittent, such as for auto emission control, the catalytic convertersystem undergoes a warmup period prior to its effective operation.Obviously, the warmup period is not as effective in terms of the desiredreaction as the steady-state hot condition. For these reasons, thecatalyst support substrate should have a relatively low heat capacity sothat its heats up quickly during start-up. The warmup characteristics ofthe catalyst support substrate are illustrated for the automotivecatalytic converter in Table 5. The all-alumina system of Table 5contained two forms of alumina; the structural framework composedprimarily of alpha-alumina and a layer of transition alumina which wouldserve as porous catalyst support. The nickel transition aluminacomposite included a reinforcing screen which will be described in moredetail hereinbelow.

                  Table 5                                                         ______________________________________                                                                     Nickel-Alumina                                   System            All-Alumina                                                                              Composite                                        ______________________________________                                        Cylindrical Monolith                                                                            7.6 cm     7.6 cm Length                                    Dimension (888 cm.sup.3 Volume)                                                                 Length ×                                                                           × 12.2 cm                                                    12.2 cm    Diameter                                                           Diameter                                                    Component Weight (gm)                                                          Transition Alumina                                                                              50        150                                               Structural Alumina                                                                             450        --                                                Metal Powder     --         350                                               Screen           --         350                                              Total Weight (gm) 500        850                                              Monolith Density (gm/cm.sup.3)                                                                  0.56       0.96                                             Monolith Primary Area (m.sup.2 /m.sup.3)                                                        3149       1050                                             Monolith Heat Capacity,                                                       (20° C.), cal/gm/° C.                                                             0.18        0.123                                           Monolith Volumetric Heat                                                      Capacity, cal/cm.sup.3 converter/° C.                                                    0.10       0.12                                             Monolith Heat Capacity Based                                                  on Transition Alumina,                                                        cal/gm/° C.                                                                              57.5       22.3                                             ______________________________________                                    

From the comparison to the all-alumina monolith shown in Table 5, it canbe seen that the volumetric heat capacity (cal/cm³ converter/°C.) isabout the same (alumina is 0.10, nickel-alumina composite is 0.12), butthat the heat capacity based on the contained transition alumina (cal/gmtransition alumina/°C., i.e., based on the available area for catalystdeposition) is considerably better (alumina is 57.5, nickel-aluminacomposite is 22.3). The very high density of the metal component isessentially compensated for (relative to alumina) by its lower specificheat capacity so that the all-alumina and metal-alumina compositevolumetric heat capacity are approximately equivalent. However, thenickel-transition alumina structure contained three times as much activecatalyst support (transition alumina) as did the all-alumina structureof the same dimensions. Accordingly, for the same required area forcatalyst deposition, the composite material monolith requires lessvolume and thereby has better warmup characteristics. It should beunderstood that the low heat capacity requirement is important only forintermittent systems and would not be important for primarilysteady-state operations. Also, the fact that the primary surface area ofthe nickel-alumina substrate of this invention is considerably less thanthat of the all-alumina substrate reflects the easy accessibility of alarge intra-pore volume of the former due to its substantial, openinter-pore system.

In a preferred embodiment of the invention, a metal screen, grid, meshor perforated metal sheet is used to give the composite structureadditional mechanical strength. (The screen, grid, mesh or perforatedmetal sheet materials will hereinafter be referred to generically as"screen.") The mixture of metal and transition alumina powders arespread over and compacted into the screen prior to sintering and thesintering step causes direct bonding of the metal component to thescreen. The result is an extremely strong porous composite material. Thescreen imparts considerable additional strength and flexibility to thecomposite that makes it very resistive to mechanical loads such asvibration, and the relative ductility of the composite makes it possibleto form the material into desirable structures, such as those describedlater with reference to FIGS. 2 and 3. While the non-reinforced catalystsupport substrate is ideally suited for forming into pellets for use inpacked beds, the screen reinforced substrate is ideally suited forforming into flat, relatively thin sheets for use in various stacked orwoven structures.

It is important to note that for the automobile exhaust catalystembodiment the catalyst support substrate is primarily compacted intothe screen between the wire grid. That is, the substrate is not just acoating on the screen, and the thickness of compacted substrate does notsubstantially exceed the screen thickness. That arrangement results in avery effective and strong catalyst support system. Essentially, eachunit of substrate within the screen grid is an independent unit andindividually supported on all four sides. This leads to a very flexiblestructure that can be easily formed and has extremely good resistance tomechanical loads and thermal shock. Substantially all the substratewithin the grid support is active as a catalyst support (as opposed toprimary area surface only coatings) and leads to a system that has ahigh surface area per unit volume.

The thickness of the sintered screen reinforced catalyst supportsubstrate system will be about twice the final (after compaction)cross-sectional dimension of the screen wire, since the limiting elementduring compaction will be the screen wire overlays. This does mean thatthe substrate thickness will exceed the wire diameter in certain regionsof the grid but only by about the wire dimension. Further, in mostregions, the "excess" substrate thickness is distributed on both sidesof the screen wire, leading to a very effective support structure inwhich each element of catalyst support substrate is essentiallyindependently supported by the surrounding screen wires. There isrelatively little interaction between adjacent elements as a result ofportions of substrate which overlap the screen wire. This would not bethe case if the screen reinforced system was substantially thicker thantwice the wire diameter. An ideal screen support would be a grid formedof circular wires all in the same plane, but the woven screen is a morepractical and economical version of the ideal support structure.

As catalyst support systems for use in auto emission control, the screenreinforced substrate systems should preferably be between about 0.25 and1.25 mm thick (and most preferably not greater than 0.75 mm thick) withcorresponding wire mesh diameters of about 0.125 to 0.625 mm. The gridopenings of the wire mesh should preferably be about 0.50 to 2.5 mm,respectively. The preferred dimensions reflect the fact that thickersubstrate systems tend to lose ductility and are not as effective inauto emission control service from the standpoint of utilization of thecatalyst on all of the supporting alumina (particularly the particleswithin the inner regions of the substrate), while substrates thinnerthan the preferred dimensions make monolith structures unduly complexand costly due to the need for supplying, adequately supporting, andspacing a relatively larger number of layers in a monolith in order toprovide the required amount of catalyst per unit volume. The preferreddimensions for catalyst support systems for use in services other thanauto emission control, of course, vary widely depending on theapplication, but the preferred dimensions for each type of service canbe easily determined according to standard engineering practices.Suitable wire screen materials are the same as those that satisfy thecriteria described hereinabove for the metal component of thenon-reinforced substrate.

A factor to be considered in the formation of the catalyst supportsubstrate is the compaction pressure. In general, the higher thecompaction pressure on the mixed powder components, the higher will bethe tensile strength of the sintered substrate. However, greatercompaction pressure reduces inter-particle porosity so that the degreeof compaction must be chosen to accommodate both strength and porosityrequirements. Tests were conducted with substrates prepared with70%--310 stainless steel powder and 30% transition alumina, and withoutreinforcement. Specimens compacted to 2800 kg/cm² averaged 375 psi (26kg/cm²) ultimate tensile strength, while those compacted to 5600 kg/cm²averaged 740 psi (52 kg/cm²) tensile strength. Whereas strength andporosity requirements may vary among specific applications, compactionto about 5600 kg/cm² has been found to provide an effective balancebetween those two factors for most uses of the 70/30 stainlesssteel/alumina substrate.

The preferred range of compaction pressures will of course vary withfactors such as the specific metal or metals employed, the ratio ofmetal-to-alumina, and the use of other components such as transitionmetal oxides. Another factor is the optional use of a vehicle orlubricant for the particles, for example, water or methanol. If desired,a plastic binder dissolved in a suitable solvent may be employed, forexample, an isobutylene polymer with molecular weight of about 140,000(commercially available under the name Vistanex) dissolved in kerosene.The use of such liquid components tends to reduce the compactionpressure required. The useful range of compaction pressures can beeasily determined for any given system by conducting appropriate testson a few samples produced with varying degrees of compaction.

When screen reinforcement is employed to produce a substrate forautomobile exhaust catalyst service, it is preferred that themetal-alumina powder be applied in such quantity per unit area ofscreen, that when the selected degree of compaction is applied, thepowder thickness is reduced to about the thickness of the screen. Thiswill produce the preferred structure described previously, wherein themetal-alumina substrate is wholly contained within the grid of thescreen. For example, using type 304 stainless steel screen, 7.9meshes/cm, 0.24 mm wire, and nickel transition alumina powders (70 wt. %Ni) the appropriate ratio, powder/screen, for a compaction pressure of5600 kg/cm² is about 0.10 gm. powder per square centimeter screen. Theresultant compact has a thickness of about 0.50 cm which is essentiallyequal to the original thickness of the screen. The ultimate strength ofsuch screen-reinforced substrates is not measurably affected by degreeof compaction, provided that the reinforcement is not significantlydeformed in the process. Substrates prepared using the materials andcomposition ratios of the above example were compressed to varyingdegrees and sintered at 870° C. for 6 hours in a hydrogen atmosphere.The data given in Table 6 show that overall strength is independent ofcompaction pressure. Therefore, within a broad range of usefulcompaction pressures, the degree of compaction may be selected solely tofacilitate optimization of strength, loading and performance of theporous substrate secured within the screen grid.

                  Table 6                                                         ______________________________________                                                 Compaction Pressure                                                                            Tensile Strength                                    Specimen kg/cm.sup.2      kg/cm.sup.2                                         ______________________________________                                        A        2812             562                                                 B        5624             562                                                 C        11248            562                                                 ______________________________________                                    

Additional mechanical strength can also be imparted to the compositestructure of this invention by means of supports other than metalscreens; for example, metal fibers such as steel wool, and solid metalsurfaces such as flat metal sheets, round metal tubes and fluted metaltubes. The composite structure of this invention can also be made in theform of tablets or pellets, these forms of unsupported composite beingparticularly useful in packed bed reactors.

Where the composite structure of this invention is supported by metalfibers the process for producing the supported structure issubstantially the same as with support screens, that is, the mixture ofmetal powder and transition alumina powder is mixed and deposited aroundthe metal fibers with sufficient packing or tamping to eliminate grossvoids, and the three component system is then compacted into the desiredshape and sintered.

Where a solid metal surface is used as the support for the sinteredcomposite, it is preferable to prepare the metal powder-transitionalumina powder mixture in the form of a slurry which also includes anorganic solvent and a liquid plastic binder material, and then apply theslurry as a coating to the base metal by dipping or spraying. Theresulting coating is then air dried and the bulk of the solvent removedby evaporation, leaving a self-supporting layer of the metalpowder-alumina powder mix which is held in place by the remainingbinder. The solid metal and coating are then heated to decompose theresidual organic material and to effect sintering of the metal powder.In using the slurry method, the metal powder-alumina powder combinationgenerally undergoes sufficient compaction during application of theslurry to retain its desired shape as a layer on the metal surface, orsome additional compaction can be provided by mechanically compressingthe coating after evaporation of the solvent and before the heating andsintering step.

The above-described method for applying a sintered coating to a metalsurface, including details as to solvents and organic binder materials,is disclosed in greater detail in U.S. Pat. No. 3,384,154 to Milton. Atwo step method for applying a sintered metal coating and which can alsobe used in the method of this invention is described in U.S. Pat. No.3,753,757 to Rodgers, et al.

The porous composite structure of this invention can be used in avariety of structural sizes and shapes. Two structures particularlyuseful for automobile catalytic converts are shown in FIGS. 2 and 3.

FIG. 2 illustrates a catalytic converter in which metal reinforcedcatalyst support substrates of this invention are helically wound toproduce a monolithic mass. FIG. 2 (a) is an isometric exploded viewshowing the method for carrying out helical winding. A flat strip 10 ofmetal screen reinforced catalyst support substrate and a corrugatedstrip 12 of metal screen reinforced catalyst support substrate arepreferably stapled together and wound about a solid cylindrical core 14in the direction of arrow 16 to produce a cylindrical monolithic massshown in isometric view in FIG. 2 (b) in which the alternating layers ofsmooth 10 and corrugated 12 catalyst support surround the central core14. FIG. 2 (c) is an elevational view of an automobile catalyticconverter containing the monolithic structure of FIG. 2 (b). Thecatalytic converter comprises a hollow metal shell 18 which includes acylindrical center portion 20 which tapers to gas inlet 22 at one endand gas outlet 24 at the opposite end. The monolithic mass of FIG. 2 (b)is disposed within the hollow center portion 20 of shell 18 with theaxis of the cylindrical core 14 coincident with the longitudinal axis ofshell 18. If desired, conventional thermal insulation can be providedaround the monolith to reduce heat loss and shorten the warm-up period.In this configuration the gas flow passages 26 which are defined by thespaces between the flat strip and the corrugated strip are orientedparallel to the flow of exhaust gas through the converter, resulting ingood gas flow and pressure drop characteristics. This configuration alsoreduces the possibility of blockage of the gas passages by particulatematter in the exhaust gas stream.

In a typical converter structure of the type shown in FIG. 2 the flatstrip 10 is about 0.4 mm thick and 7.6 cm wide and the corrugated strip12 is also 7.6 cm wide, approximately 0.4 mm thick prior to corrugationand approximately 3.7 mm thick in its corrugated form. The core 14 is7.6 cm long and about 2 cm in diameter. About 2.5 meters of the flatstrip 10 and about 5 meters of the flat strip in corrugated form 12 areneeded to produce a monolithic mass of FIG. 2 (a) which is 7.6 cm longand about 12 cm in diameter, and in which the gas flow passages 26 are7.6 cm long and approximately 3.7 mm in their maximum cross-sectionaldimension in the radial direction.

The corrugations, such as those in strip 12 described above, can beformed either before or after the sintering step by conventional methodssuch as passing the strip between grooved rollers or by embossment undera stamping machine. Generally only moderate pressure is required to formadequate corrugations, and excessive pressure should be avoided toreduce the possibility that surface pores will be wholly or partiallyclosed, thus inhibiting gas flow into the sintered matrix.

Another suitable structure for an automobile catalytic converter isshown in FIG. 3.

The catalyst support structure for the converter of FIG. 3 is made up ofa series of disc shaped elements 30 composed of the porous compositestructure of this invention. The discs 30 have an opening 32 in centerand a series of radially disposed corrugations 34.

To form the monolithic mass shown in isometric view in FIG. 3 (b), aseries of individual discs 30 are stacked one against the other with theopenings 32 in axial alignment and with the corrugations 34 on each disc30 resting against a flat portion 36 of the adjacent element so thateach disc is spaced away from the next adjoining disc. The openingsdefined by the flat surfaces 36 and 38 and corrugations 34 provide gaspassages 40 through the monolithic mass.

FIG. 3 (c) is a cross-sectional view of an automobile catalyticconverter containing the monolithic mass of FIG. 3 (b). The convertercomprises a hollow cylindrical shell 42 having a relatively long centerportion 44 which tapers to gas inlet 46 at one end and gas outlet 48 atthe opposite end. The monolithic mass of FIG. 3 (b) is disposed in thecenter section 44 of shell 42 with the large passageway 49 formed by thecentral openings 32 in the discs 30 coincident with the longitudinalaxis of the shell 42. A circular baffle 50 having a central opening 52is disposed at the inlet end of shell 42 and a circular baffle 54 havinga solid center portion 56 and an annular opening 58 is disposed at thegas outlet end of shell 42. Exhaust gas entering the converter isdeflected by baffles 50 and 54 and passes through the many gas passages40 before leaving the converter through opening 58 and gas outlet 48.The solid arrows 60 illustrate the typical pattern of gas flow.

In a typical converter structure of FIG. 3 each disc 30 is approximately7.6 cm in diameter and 0.4 mm thick with a center opening 32 which is2.5 cm in diameter and corrugations 34 which extend 0.64 mm above thesurface of the disc. About 200 of the discs 30 when stacked together asshown in FIG. 3 (b) form a monolithic mass about 7.6 cm in diameter andabout 20 cm long.

The specific dimensions of the catalyst support will be determined bythe requirements of the particular application, but there are somedesign criteria that apply to many applications. The monolothicstructure appropriate for auto emission control uses should be arrangedso that it has a primary surface area (i.e., the boundaryfluid-substrate interface) of at least 325 m² /m³ of monolith whichassures sufficient area to allow ready access of the fluid to the innerregions of the substrate where most of the active area is contained. Fora structure such as that shown in FIG. 3 (b) it has been found that thelayer-to-layer spacing should be 1.3 cm or less and the number ofcorrugations in the alternate layers should be at least 0.8corrugations/cm.

The total size and primary surface area of the monolithic mass will bedetermined by the available pressure drop for the given application andthe required area. The latter limitation is obtained from theoperational space velocity (i.e., the volume of process gas/unittime/volume of catalytic converter). The available system pressure dropwill determine the geometric shape of the catalytic converter. That is,a large available pressure drop will favor a structure with a small facearea and long flow path, whereas a small available pressure drop willfavor large face area and short flow path.

When the porous composite structure of this invention is to be used as acatalyst support substrate, there are several methods of catalystaddition that can be used to deposit the desired catalyst on thesupport. In one method, the catalyst is added to the components of thecatalyst support substrate during its manufacture, whereas in a secondmethod, the catalyst is deposited on the support after its manufacture.These methods can be referred to as pre-deposition and post-depositiontechniques, respectively.

With the pre-deposition method, the catalyst is dispersed onto thetransition alumina prior to the incorporation of the alumina into thecatalyst support substrate. The deposition technique involves mixingsalts of the catalysts (usually nitrates) in the desired proportions,dissolving them into an aqueous solution, mixing the transition aluminainto the solution and then evaporating the water. If the active form ofthe catalyst is the metal or metal oxide an additional step of heatingthe mass in a hydrogen, air or other suitable atmosphere to convert thesalts into the catalysts (usually metals and/or metal oxides) and toseparate the off gases. The advantages of the pre-deposition techniqueare that it is relatively easy to bring all the transition alumina intointimate contact with the catalyst salt solution, thereby assuring evendistribution of the catalyst upon the surface area of the transitionalumina. The major disadvantage of the technique is that the catalystsare exposed to the sintering process during the subsequent manufactureof the catalyst support substrate. Exposure of the catalysts to therelatively high temperatures (up to 982° C.) may in some instancesreduce catalyst activity.

With the post-deposition technique, the catalyst salt solution is usedto saturate the already formed catalyst support substrate and then themass is heated, as in the pre-deposition technique, in a suitableatmosphere to convert the salts into the catalyst. The advantage of thepost deposition method is that it avoids exposing the catalyst to thesintering temperatures. The disadvantages of the post-deposition methodare that catalyst is deposited on both the metal and transition aluminacomponents of the substrate (although this is not a serious drawback dueto the much higher surface area of the transition alumina) and that itis not always possible to assure even distribution of the catalyst onthe transition alumina since the substrate is a rigid porous structureand would depend on capillary action to reach the inner regions of thesubstrate.

In some instances it may be desirable to use a combination of both pre-and post-deposition techniques. For example, if a two component catalystis to be employed, one component of which is sensitive to heat while theother component is not, the post-deposition technique can be used forthe heat-sensitive catalyst component and the pre-deposition techniquefor the other catalyst component.

Thus, the catalyst deposition technique to be used depends on theproperties of the individual catalysts and the conditions to be used inthe manufacture of the catalyst support substrate and the most efficienttechnique can be easily determined for any given catalyst or catalystcombination.

In addition to its use as an automobile emission control apparatus, thecatalyst support substrate of this invention is also useful in sulfurremoval from flue gases, control of air pollution with after-burners,and other catalytic reactions, for example, any application requiringany of the following characteristics: high mechanical strength, goodthermal stability, non-fouling characteristics, low pressure drop, oreffective utilization of the active catalytic component supported on thesubstrate.

The catalyst support substrate of this invention can be used in theremoval of sulfur dixoide from flue gas by employing copper oxide as thecatalyst. The sulfur dioxide is removed from the gas stream as a resultof the following chemical reaction:

    CuO+SO.sub.2 +1/2O.sub.2 →CuSO.sub.4

The copper oxide is regenerated by a two step process of heating firstin hydrogen and then in oxygen. The regeneration reactions arerepresented by the following equations:

    CuSO.sub.4 +2H.sub.2 →Cu+SO.sub.2 +2H.sub.2 O

    Cu+1/2O.sub.2 →CuO

Copper oxide deposited on the catalyst support substrate of thisinvention is also useful in the removal of hydrogen sulfide from fuelgas. The chemical reaction for H₂ S removal is the following:

    2Cu+H.sub.2 S→Cu.sub.2 S+H.sub.2

Regeneration of the copper oxide is then carried out by heating first inoxygen and then in hydrogen. The regeneration reactions are representedby the following equations:

    2Cu.sub.2 S+5O.sub.2 →2CuO+2CuSO.sub.4

    CuO+CuSO.sub.4 +3 H.sub.2 →2Cu+SO.sub.2 +3H.sub.2 O

In the above described reactions for the removal of sulfur dioxide fromflue gas and hydrogen sulfide from fuel gas, the copper oxide functionsmore as a chemisorbent than a catalyst, in that it is consumed in thereaction and has to be periodically regenerated. Thus, for the purposesof this invention, the term "catalyst" includes both conventionalcatalysts which are not consumed in the course of a catalytic processand chemisorbents which require regeneration from time to time.

The method of depositing copper oxide or other chemisorbent on thecatalyst support substrate of this invention is substantially the sameas for any of the other catalyst materials, and either thepre-deposition or post-deposition techniques described hereinabove canbe employed.

The following more detailed examples further illustrate the invention.

Example I

This example employed pre-deposition of a platinum catalyst on thetransition alumina. The catalyst-loaded alumina was combined with nickeland then formed into pellets and sintered. A packed bed of the pelletswas used in tests to demonstrate its effectiveness in removing oxidationtype pollutants (i.e., hydrocarbons and carbon monoxide) from automotiveexhaust gases.

The platinum catalyst was pre-deposited on the transition alumina bydissolving 2.66 gm of H₂ PtCl₆.6H₂ O (equivalent to 1 gm of Pt metal) in50 cc water, adding 99 gm of transition alumina (Alumina #1 of Table 2,325 mesh size, i.e. 128 meshes/cm) and heating the mass at 110° C. for24 hours. The mass was then further heated at 300° C. in an atmosphere(15% H₂ in N₂) designed to reduce the catalyst salt to the desiredplatinum metal. After heating for 24 hours, the mass was cooled toambient temperature in a nitrogen atmosphere.

After cooling, the catalyst-loaded transition was ground to a 325 meshsize and combined with fine nickel powder (400 mesh size, i.e. 158meshes/cm) in the proportions of 20 gm alumina and 80 gm nickel. Themixture was uniformly blended in a ball-mixing device and then compactedinto 4.76 mm diameter pellets, 3.2 mm long by means of a conventionalpelletizing apparatus. The pellets were sintered in a H₂ atmosphere for6 hours at 870° C. After cooling, the pellets were ready to be used inthe catalytic converter. The pellets had a surface area of 29 m² /gm andbulk density of about 1.9 gm/cm³. A simulated automobile exhaust streamcomposed of CO--0.8%, C₃ H₆ --400 ppm, O₂ --1%, NO--1380 ppm,helium--balance, was passed through a bed of these pellets 2.5 cm longand 2.5 cm in diameter at a bed inlet temperature of 500° C. Thefollowing results were obtained:

    ______________________________________                                        Bed Space Velocity    CO Conversion, %                                        ______________________________________                                        (cm.sup.3 gas (NTP)*/hr/cm.sup.3                                              bed volume)                                                                   17,000                89                                                      33,000                88                                                      110,000               78                                                      ______________________________________                                         *NTP = normal temperature and pressure = 20° C. and 760 mm H.sub.2

These results show good performance of the reactor system.

Example II

This example illustrates the use of a catalyst support substrateprepared by pre-loading of a noble metal-promoted multi-componenttransition-metal oxide catalyst on transition alumina. Thecatalyst-loaded alumina was then combined with nickel powder, compactedinto a stainless steel screen, and sintered to form a reinforcedcatalyst support substrate already loaded with the catalyst. Thereinforced substrate was then stacked in a monolith that was used as acatalytic converter in a simulated automobile exhaust.

A catalyst solution was prepared by combining the following:

5 gm. Cu as 19.1 gm. of Cu(NO₃)₂.3H₂ O

5 gm. Mn as 38.2 gm. of 50% aqueous Mn(NO₃)₂

5 gm. Cr as 38.4 gm. of Cr(NO₃)₃.(H₂ O)

0.063 gm. Pd as 1.35 gm. of 10% aqueous Pd(NO₃)₂

and dissolving them in 20 cc of water. The transition alumina (85 gm. ofAlumina #1 of Table 2, 325 mesh size) was then added and the entire masswas mixed and heated to drive off the water. The mass was then broken upand heated in an atmosphere (15% H₂ in N₂) to reduce the catalyst saltsto the metal oxides. The catalyst-loaded alumina was then combined withfine nickel powder (400 mesh size) in the proportion of 30 gm.catalyst-loaded alumina and 70 gm. nickel powder. This mixture wascompacted by means of a hydraulic press into a 310 Stainless Steelscreen (18×18 mesh size, i.e. 7.1 meshes/cm, 0.23 mm wire). The powdermixture was used at a loosely packed depth of 1.0 mm and the screen wascompacted into it at a pressure of 5625 kg/cm² to a composite thicknessof 0.5 mm. After compaction, the green substrate was sintered in ahydrogen atmosphere at 870° C. for 6 hours. After sintering eleven testsstrips (0.97 cm×5 cm×0.5 mm) were stacked with alternate strips of blankscreen to form a test unit in the form of a stacked monolith. The testmonolith was enclosed in a suitable shell so that it could be heatedwith a tubular electric furnace to the desired operation temperature.Reactant inlet gas was introduced into the test unit and flowed paralleland through the major length of the monolith. The inlet gas (composed ofprepared mixtures) was preheated to the desired inlet temperature andflow regulated to obtain the desired space velocity for the catalyticconverter. The outlet gas was analyzed using a mass spectograph. Theinlet gas had a simulated automobile exhaust composition of CO--0.8%, C₃H₆ --400 ppm, O₂ --1%, NO--1380 ppm, 10 wt.% water, helium--balance. Thefollowing results were obtained at an inlet gas temperature of 500° C.

    ______________________________________                                        Converter Space Velocity                                                                            CO Conversion, %                                        ______________________________________                                        (cm.sup.3 gas (NTP)/hr/cm.sup.3                                               converter volume)                                                             17,300                92                                                      34,600                90                                                      120,000               65                                                      ______________________________________                                    

Example III

In this example the substrate preparation and test procedures of ExampleII were followed except that 310 Stainless Steel powder was used inplace of nickel powder and the sintering step was carried out at 980° C.for 6 hours. The following results were obtained:

    ______________________________________                                        Converter Space Velocity                                                                            CO Conversion, %                                        ______________________________________                                        (cm.sup.3 gas (NTP)/hr/cm.sup.3                                               converter volume)                                                             17,000                91                                                      34,000                90                                                      100,000               45                                                      ______________________________________                                    

Example IV

This example employed a commercially available pre-deposited platinumcatalyst on transition alumina containining 0.5 wt.% platinum. Thecatalyst was combined with 310 stainless steel powder in the proportions30 gm catalyst and 70 gm metal powders and compacted into a 310stainless steel screen as in Example II except the 0.95 cm×5 cm×0.5 mmstrips after compaction was sintered in hydrogen atmosphere at 927° C.for 3 hours, and was tested in the same was as Example III except thatinlet gas had a simulated automobile exhaust composition of CO--1%, C₃H₆ --300 ppm, O₂ --1%, water vapor--10 wt.%, nitrogen-balance gaveresults as follows:

    ______________________________________                                        Converter Space Velocity                                                                            CO Conversion, %                                        ______________________________________                                        (cm.sup.2 gas (NTP)/hr/cm.sup.2                                               converter volume)                                                             17,000                98                                                      34,000                92                                                      100,000               74                                                      ______________________________________                                    

Example V

This example was carried out in the same way as Example IV exceptplatinum catalyst (0.5 wt.-% Pt) was prepared as described in Example I.The following results were obtained:

    ______________________________________                                        Converter Space                                                                              CO           C.sub.3 H.sub.6                                   Velocity       Conversion, %                                                                              Conversion, %                                     ______________________________________                                        (cm.sup.2 gas (NTP)/hr/                                                       cm.sup.2 converter volume)                                                    17,000         98           --                                                34,000         88           90                                                100,000        80           78                                                ______________________________________                                    

Example VI

This example illustrates the preparation and use of a post-depositednoble metal-promoted multi-component transition-metal oxide catalyst ona catalyst support substrate prepared by combining 310 stainless steelpowder and transition alumina in the proportions of 70 gm of 310stainless steel powder and 30 gm of transition alumina. The mixture wascompacted by means of a hydraulic press into a 310 stainless steelscreen (18×18 mesh, 0.23 mm wire) as in Example II. After compaction,the green substrate was sintered in a hydrogen atmosphere at 929° C. for3 hours. The sintered catalyst support strips (0.95 cm×5 cm×0.5 mm) werethen loaded with catalyst from a solution containing:

5.3 gm Cu (NO₃)₂.3H₂ O

9.1 gm Mn (NO₃)₂ -50% aqueous

10.8 gm Cr (NO₃)₂.6H₂ O

0.061 gm Pd (NO₃)₂

26.8 gm methanol (35 cc)

by dipping and drying. Two successive applications of catalyst solutiongave an approximate catalyst composition based upon transition aluminaof 5 wt.% Cu, 5 wt.% Mn, 5 wt.% Cr and 0.1 wt.% Pd as oxides afterremoval of water, methanol and oxides of nitrogen. Tested in the samemanner as Example V, the following results were obtained:

    ______________________________________                                        Converter Space Velocity                                                                             CO Conversion, %                                       ______________________________________                                        (cm.sup.3 gas (NTP)/hr/cm.sup.3 converter volume)                             17,000                 90                                                     34,000                 74                                                     100,000                30                                                     ______________________________________                                    

Example VII

A life test of palladium-loaded substrate of this invention wasconducted under conditions simulating automotive exhaust service. Thesubstrate was prepared and loaded as follows:

A quantity of -325 mesh transition alumina (Alumina #2 of Table 2) waspretreated by heating, first in a hydrogen atmosphere at 927° C. for 3hours, then in steam at 871° C. overnight. The alumina was then blendedwith -325 mesh 310L stainless steel in proportions 70 wt.% stainlesssteel, 30 wt. % alumina and mixed thoroughly. The powder was compactedat 5625 kg/cm² into 0.23 mm wire 18×18 mesh 310 stainless steel screenand sintered at 1700° F. for 3 hours in hydrogen. The finished thicknesswas about 0.5 mm.

The substrate was soaked in a solution prepared by dissolving 1.3 gmPd(NO₃)₂ in 50 cc methanol and the absorption of solution by thesubstrate was such as to infuse 0.8% by weight elemental palladium basedon the weight of alumina. The wet substrate was dried and then heated inair at 816° C. for 18 hours to decompose the the nitrate and form thePdO catalyst.

Nine strips of the catalyst-loaded substrate each 9.5 mm wide 25 mm longwere stacked with 0.5 mm diameter chromium steel wire spacers and fittedinto a rectangular refractory tube 14 mm×11 mm inside dimensions. Thetube was installed in a furnace and connected to sources of gas mixturesto be described.

The following series of tests were conducted to determine catalystactivity, surface area stability, and structural durability.

    ______________________________________                                        (a) Lightoff tests:                                                                             A gas mixture was prepared of                                                 the following composition:                                              CO         0.5%                                                               O.sub.2    1.0%                                                               H.sub.2    0.17%                                                              Hydrocarbon                                                                              150 ppm                                                            (Propane)                                                                     H.sub.2 O  Saturated at 50-                                                              51° C.                                                      N.sub.2    Balance                                                            This mixture was passed through the                                           tube at a space velocity (cm.sup.3 gas                                        (NTP)/hr/cm.sup.3 bed volume) of 34,000                                       while the temperature was gradually                                           increased. The temperature at which                                           50% of the influent CO was removed                                            was recorded by means of an infra-                                            red analyzer as the lightoff tempera-                                         ture.                                                             (b) CO Removal tests:                                                                           A gas mixture                                                                 was prepared of the                                                           following composition:                                                  CO         1%                                                                 O.sub.2    1%                                                                 H.sub.2    .33%                                                               Hydrocarbon                                                                              300 ppm                                                            (Propane)                                                                     N.sub.2    Balance                                                            This mixture was passed through the                                           tube at variable space velocity                                               while holding the effluent tempera-                                           ture at 500° C. After equilibration,                                   the percent removal of CO was re-                                             corded and occasionally, the per-                                             cent removal of hydrocarbons. Space                                           velocities (same as test (a) above)                                           of 17,000, 34,000 and 100,000 were                                            applied.                                                          (c) Steaming tests:                                                                             A gas mixture was prepared of the                                             following composition:                                                  O.sub.2     1% (dry basis)                                                    H.sub.2 O  10% (wet basis)                                                    N.sub.2    Balance                                                            This mixture was passed through the                                           tube for 16 hours with the furnace                                            temperature held at 816° C. The                                        results of lightoff and CO removal                                            tests were compared to quantify                                               the effect of steaming.                                           (d) Durability tests:                                                                           The gas mixture described for the                                             CO removal tests was passed through                                           the tube at steady space velocity                                             (same as test (a) above) of 10,000                                            with the furnace adjusted to maintain                                         an effluent temperature of                                                    500° C. After selected intervals of                                    time, space velocities were                                                   momentarily increased to prescribed                                           values for CO -removal determinations                                         After 420 hours operation, 15 ppm                                             SO.sub.2 was continually added to the gas                                     mixture to evaluate catalyst resis-                                           tance to sulfur poisoning. This was                                           increased to 25 ppm after 550 hours                                           and continued through the remainder                                           of the life test. The duration of                                             the test was 2200 hours, equivalent                                           to about 55,000 miles of normal                                               service in an automobile.                                   ______________________________________                                    

The effectiveness of the catalyst loaded substrate of this inventionremain substantially the same during the entire 2,000 hour period. Witha space velocity of 34,000 between 80% and 90% of the carbon monoxidewas removed and at a space velocity of 100,000 between about 65% and 75%of the carbon monoxide was removed. About 80% of the hydrocarbon contentof the gas stream was removed.

The activity of the catalytic system was reduced for a short period oftime immediately following steaming, but the activity was fully restoredafter the steaming tests were completed.

The addition of sulfur dioxide as described above had no detectableeffect on catalyst activity.

During the third steaming test, the oxygen content of the gas wasinadvertently increased to an unknown higher value for about 16 hourswithout any detectable decrease in the performance of the catalystloaded substrate.

Lightoff determinations made during the series of steaming tests aresummarized in the following Table. The low lightoff values attest to ahigh surface area substrate. The stability of the lightoff values duringsteaming tests shows that high surface area was maintained and that nodeleterious interactions occurred between substrate and catalyst. Thesustained low values of lightoff and high values of CO removalthroughout the life test are further evidence of an extremely stablesurface area. Following the life tests, inspection showed that thesubstrate had retained ductility and that the sintered matrix was stillsecurely held within the screen reinforcement.

    ______________________________________                                        Catalyst-Substrate     Lightoff                                               Condition              (° C.)                                          ______________________________________                                        Fresh                  <164°                                           After 1st Steaming     <167°                                           After 2nd Steaming     <168°                                           After 3rd Steaming     <195°                                           After 4th Steaming     <182°                                           ______________________________________                                    

The strips of catalyst-loaded substrate used in the foregoing test wereoriginally sheared from larger pieces of substrate, and the scrapmaterial was saved for "before" and "after" comparison of substrateproperties. Surface area measurements (surface area based on substratedoes not include the reinforcing screen) made on the fresh, unusedmaterial and on the strips which had completed the 2000-hour test showedthe following:

    ______________________________________                                               Surface Area  Surface Area                                                    Based on Substrate                                                                          Based on Alumina                                         ______________________________________                                        Before   25 m.sup.2 /gm  ˜ 80 m.sup.2 /gm                               After    38 m.sup.2 /gm  ˜ 125 m.sup.2 /gm                              ______________________________________                                    

These data show that, within the experimental error of the surface areameasurements, there was no significant change in the surface area duringthe 2000-hour tests.

Example VIII

A full-scale automotive exhaust reactor was built and tested on anautomobile engine. A description of construction and preparation of thereactor follows:

Quantities of 310L stainless steel powder and transition alumina(Alumina #2 of Table 2) were separately screened through a 325 meshsieve. Pre-treatment of alumina was accomplished by heating the powderin nitrogen at 927° C. and then heating in steam at 871° C. overnight.The powders were blended in proportions 70 wt.% stainless steel powderand 30 wt.% alumina and then thoroughly mixed. This dry mixture wascompacted at 5625 kg/cm² into a 3-inch wide strip of 310 stainless steelscreen, 18×18 mesh×0.23 mm diameter wire. The powder-loaded screen wasthen sintered at 927° C. for 3 hours in a hydrogen atmosphere.

After sintering, the substrate was loaded with palladium oxide catalystby soaking the strip at normal temperature and pressure in a solution ofpalladium nitrate in methanol followed by air-drying. The absorptivityof the substrate for the solution was predetermined, and the strength ofthe solution was adjusted so that in two steps of soaking and drying,the amount of elemental palladium infused into the substrate was 1% ofthe weight of the alumina component of the substrate. The air-driedstrip was heated 16 hours in air at 816° C. in order to decompose thenitrate and form the PdO catalyst in the support.

About 9.75 meters of the catalyst-loaded substrate was wound around a 19mm O.D.×0.7 mm wall nickel steel tube 7.5 cm long. Two 12.5 mm widestrips of the same stainless steel screen without powder-loading werewound together with the 7.5 cm wide strip, one at each edge of the roll,and served as spacers between laps. For the last three laps, solid 12.5mm wide strips of 310 stainless steel 0.3 mm thick were substituted forthe 12.5 mm wide screen strips to serve as outer bindings, the ends ofwhich were welded to secure the coil. The finished coil was 12 cmdiameter and weighed 1.2 kg. The coil was fitted into a flanged casingand sealed by bolting to form a complete reactor of the general shapeshown in FIG. 2 (c).

The completed reactor was installed on a stationary 1971 Ford V-8engine, 4900 cm³ displacement, so that it received exhaust gas from oneside of the engine (4 cylinders). The engine was run on premiumnon-leaded gasoline. On the other side of the engine, receiving theexhaust from the opposite 4 cylinders was an all-ceramic monolithiccatalytic reactor composed primarily of alpha-alumina coated withtransition alumina and loaded with platinum catalyst. Results of the80-hour test are summarized in Table below. The pressure drop wasmeasured across the reactors by pressure taps placed in closely adjacentconnecting piping.

Results show that from the standpoint of catalyst compatibility, thesubstance of this invention is as good or better as a catalyst supportunder service conditions than the highly porous, all-ceramic substrate.

    __________________________________________________________________________                         CO Composition (%)                                               Engine       No Air        Air Addition  Pressure                             Speed                                                                              T in                                                                              T out       In-Out, %     In-Out %                                                                            Drop                         Reactor (RPM)                                                                              (° C.)                                                                     (° C.)                                                                     In  Out In, % In  Out In, % (mm Hg)                      __________________________________________________________________________    Ceramic 1500 410 --  .30 .02 93    .3  .03 90    --                           This invention                                                                        1500 410 427 .28 .02 93    .28 .02 93    --                           Ceramic 1500 441 --  .40 .04 90    .48 .03 94     5.2                         This invention                                                                        1500 437 454 .45 .01 98    .48 .01 98     4.5                         Ceramic 2500 627 693 1.34                                                                              1.00                                                                              25    1.35                                                                              .57 58    --                           This invention                                                                        2500 599 688 1.60                                                                              1.15                                                                              28    1.60                                                                              .54 66    --                           Ceramic 2500 620 688 1.46                                                                              1.00                                                                              32    1.22                                                                              .37 70    11.6                         This invention                                                                        2500 584 666 1.54                                                                              1.06                                                                              31    1.31                                                                              .30 77    12.0                         __________________________________________________________________________

Example IX

The same reactor containing catalyst-loaded substrate of this inventionthat was employed in the engine tests of Example VIII was installed in astandard 1973 Vega (2294 cm³) displacement). A standard 1972 Vega airpump was also installed to inject air into the exhaust stream near themanifold. No choke or carburetor adjustments were made. The followingresults were obtained with and without the reactor.

    ______________________________________                                                  Grams/mile in engine exhaust                                        Component   Cold Start                                                                              Stabilized                                                                              Hot Start                                                                            Total                                  ______________________________________                                        Hydrocarbon                                                                    Without Reactor                                                                          .72       .35       .42    1.49                                    With Reactor                                                                             .15       .14       .24    .53                                     % Reduction                                                                              79%       60%       43%    64%                                    Carbon Monoxide                                                                Without Reactor                                                                          8.12      2.90      2.22   13.24                                   With Reactor                                                                             2.03      .54       .82    3.39                                    % Reduction                                                                              75%       81%       63%    74%                                    ______________________________________                                    

Example X

A sintered substrate of this invention was prepared by the same methodas the substrate of Example VII.

The sintered substrate was catalyst-loaded by dipping into 100 mlmethanol solution containing 57 gms. Cu(NO₃)₂.3H₂ O. Two dippingoperations with intervening air-drying, resulted in a loading of 15%copper (as CuO) based on alumina. The material was then heated in air at350° C. to decompose the nitrate. The CuO in this Example functions as achemisorbent.

The catalyst-loaded substrate was wound into a roll 2.5 cm diameter andthe roll was installed in a 2.5 cm ID ceramic tube. The tube wasinserted in a furnace equipped with an electric heater and was attachedto gas sources to be described.

A feed gas was prepared of the following composition: 9.6% CO₂, 0.7%SO₂, 9.03% O₂, balance N₂ (dry basis). The gas was then saturated withH₂ O at about 52° C. After preheating the furnace to 399° C., theresultant gas mixture was fed to the reactor tube at a space velocity ofabout 590 hr⁻¹ based on total volume occupied by the rolled substrate.The effluent was analyzed frequently at known intervals of time fromcommencement of feed flow. Breakthrough of SO₂ was defined as the momentwhen the SO₂ content of the effluent rose to 10% of the feed value,i.e., about 800 ppm, representing 90% removal. Under the foregoingconditions, breakthrough was observed after 30-45 min. operation.Repeated cycles of regeneration and loading did not appear to reduce theeffectiveness of the chemisorbent.

Another series of tests was conducted using a similar substrate loaded30 wt.% copper as CuO. The feed gas consisted of 1% SO₂, 10.4% CO₂,1.95% CO, 0.03% O₂, balance N₂ (dry basis), and again the gas was watersaturated at about 52° C. Breakthrough at space velocity of 590 hr⁻¹ wasobserved at times between 56 and 79 minutes, corresponding to removal ofabout 90% of the sulfur dioxide.

Example XI

Screen reinforced catalyst substrate was prepared in accordance with thespecifications of Example VIII hereinabove. Instead of palladium oxide,the substate was loaded with a copper-chromium oxide chemisorbent forremoving H₂ S from a simulated fuel gas. (The chromium oxide functionsas a dispersing agent for the copper and is not itself a chemisorbent.The sintered substrate was dipped in a solution containing 60 gm Cu(NO₃)₂.3H₂ O and 12.1 gm Cr (NO₃)₃.9H₂ O per each 100 ml methanol. Afterdrying and calcining to CuO and Cr₂ O₃, the loading based on alumina was14% by weight Cu and 1.4% by weight Cr. The loaded substrate was thenheated in a hydrogen containing atmosphere to reduce the CuO to Cumetal. Coupons of the screen-reinforced substrate 7.6 cm square werestacked with spacers to provide a bundle height of 3.2 cm.

A feed gas consisting of 2000 ppm H₂ S, 25% H₂, balance helium waspassed through the bed at a space velocity of 5500 hr⁻¹ and atemperature of 350° C. The concentration of H₂ S in the effluent wasmeasured at selected intervals of time. Results show that during thefirst portion of the cycle, through 50%- utilization of bed capacity,over 90% of the incident H₂ S was removed from the gas. When effluent H₂S reached 1000 ppm (50% removal of incident H₂ S), over 95% of the bedcapacity had been utilized and over 80% of the total H₂ S suppliedduring the cycle had been absorbed.

Example XII

A comparative test was conducted to demonstrate performance of thecatalyst support substrates of present invention for chemisorption ofsulfur dioxide, relative to an all-alumina, pelleted catalyst support. Afirst reactor was prepared using 70% by weight 310 stainless steelpowder and 30% by weight transition alumina. The mixed powders werereinforced with type 310 stainless steel screen, 18×18 mesh (7.1meshes/cm, 0.23 mm wire), sufficient powder being used to provide afinished substrate thickness of 18 mils (0.5 mm). The substrate wascompacted to 5625 kg/cm² and sintered at 870° C. for 6 hours in ahydrogen atmosphere. Twenty-four flat strips of this substrate each7.6×15.3 cm were loaded with copper in accordance with the procedure ofExample X resulting in a final content of 11.5 wt% copper (as Cu basedon alumina, or 14.4 wt% as CuO based on alumina). The strips were thenstacked with spacers to provide bundle 3.2 cm high.

A second reactor was prepared using transition alumina granules -8 to+14 mesh size (<2.38 mm, >1.41 mm). The granules were soaked in asolution of 454 gm Cu(NO₃) 3H₂ O in 900 ml methanol for 2 minutes,dried, soaked again, dried and calcined. The final content was 8.5 wt.%Cu (10.7 wt.% as CuO). Five catalyst strips were prepared eachconsisting of a layer of the above granules, each strip 7.6 cm×21.5cm×3.2 mm thick, sandwiched between flat pieces of 20 mesh stainlesssteel screen (7.9 meshes/cm). The strips were stacked with pieces of 6mesh stainless steel screen (2.36 meshes/cm) inserted between the endsto provide about 3.2 mm spacing and producing a bundle 3.2 cm high.

The two test reactors were both operated with a feed gas consisting of3000 ppm SO₂, 1.5% O₂, 6% H₂ O and N₂. The gas was introduced to eachreactor at a space velocity of 1500 hr⁻¹ and a temperature of 344° C.and measurements were made of residual SO₂ in the effluent gas atselected time intervals.

Results with the reactor containing substrate of this invention showedessentially complete removal of SO₂ for a substantial period of timeafter which the concentration rose rapidly, indicative of a sharp masstransfer front which in turn indicates effective use of the chemisorbentin the reactor. In contrast, the encaged pellet reactor produced anear-linear increase in SO₂ concentration in the effluent indicating along, diffuse mass transfer front and less effective utilization ofchemisorbent.

In both tests, the SO₂ in the effluent reached about 420 ppm after 49minutes operation, which concentration is near the maximum desirablelevel for release of stack gas from coal-fired power plants. Therefore,both substrates would have about the same on-stream time before beingremoved for regeneration. However, during the 49-minute period, theencaged pellet reactor (because of the near linear increase in SO₂ inthe effluent as described hereinabove) permitted 2.2 times as much SO₂to pass through into the atmosphere as did the reactor containing thesubstrate of this invention.

What is claimed is:
 1. A porous composite structure comprising asintered mixture of metal particles and alumina particles characterizedby the following parameters:at least 50 weight percent of the particlesin the composite are metal particles; at least 5 weight percent of theparticles in the composite are alumina particles having a surface areaof at least 25 square meters per gram; and the inter-particle porosityin the composite structure is between 5 volume percent and 60 volumepercent.
 2. The composite structure in accordance with claim 1 whereinabout 70 weight percent of the particles are metal particles.
 3. Aporous composite structure in accordance with claim 1 consistingessentially of a sintered mixture of metal particles and aluminaparticles characterized by the following parameters:at least 10 weightpercent of the particles in the composite are alumina particles having asurface area of at least 25 square meters per gram.
 4. The compositestructure in accordance with claim 3 wherein about 70 weight percent ofthe particles are metal particles.
 5. A support structure for catalyticmaterials comprising the combination of a metal screen material havingdeposited thereon a sintered mixture of metal particles and aluminaparticles, characterized by the following parameters:at least 50 weightpercent of the particles in the sintered mixture are metal particles; atleast 5 weight percent of the particles in the sintered mixture arealumina particles having a surface area of at least 25 square meters pergram; and the inter-particle porosity in the sintered mixture is between15 volume percent and 60 volume percent.
 6. A support structure forcatalytic materials in accordance with claim 5 comprising thecombination of a metal screen material having deposited thereon asintered mixture consisting essentially of metal particles and aluminaparticles characterized by the following parameters:at least 10 weightpercent of the particles in the sintered mixture are alumina particleshaving a surface area of at least 25 square meters per gram.
 7. Thesupport structure in accordance with claim 6 wherein said metalparticles are nickel or stainless steel.